Magnetic field sensors, particularly Hall sensors, are widely used for noncontact detection of positions and angles. Two configurations have basically established themselves: a bipolar arrangement and a unipolar arrangement. In the bipolar arrangement, for example, a magnetic field source such as an ordinary commercial magnet is moved along a direction x that extends parallel to the face of the magnetic field sensor. In the unipolar configuration, such a magnetic field source is moved along a direction perpendicular to the face of the magnetic field sensor.
FIG. 1A shows a typical bipolar arrangement of magnetic field sensors SM1, SM2, SM3 and a movable magnetic field source N, S. A sensor chip IC extends in a plane with an axis of motion X. The sensor chip IC comprises a first, second, third magnetic field sensor SM1, SM2, SM3 that are arranged along the axis of motion X. The magnetic field source N, S with a north pole N and a south pole S and that runs along the axis of motion X is situated above the sensor chip IC.
FIG. 1B shows the sensor arrangement from FIG. 1A viewed from the side. As a rule, the magnetic field source N, S in such arrangements is a certain distance away from the sensor chip IC. This distance is typically referred to as an air gap. This distance, as well as other parameters such as the magnetization of the magnets that are used, can be taken into account in many magnetic field sensors by calibration routines for a precise measurement.
FIG. 1C shows a characteristic magnetic field curve when the magnetic field source N, S is moved along the axis of motion X. In the figure, the magnetic field strength B is plotted versus the axis of motion X. The illustrated functional relationship corresponds, for instance, to the signal of one of the magnetic field sensors SM1, SM2, and SM3 measured with the sensor arrangement from FIGS. 1A and 1B. The combined signal is supplied to a signal processor that derives positions and angles from it.
The unipolar arrangement is not shown. With it, the magnetic field source N, S is rotated by 90° and is moved with one of the poles N, S along a direction Z that runs perpendicular to the sensor chip.
Both the unipolar and the bipolar method have in common that they must define a suitable end position for the movement of the magnetic field source N, S. Such an end position is generally reached when a given previously defined magnetic field strength or a threshold value is reached by the movement of the magnetic field source N, S. Thereby the end position becomes susceptible, however, to magnetic fields or interference fields of the type that occur in the surroundings of the magnets or electromagnetic sources during their respective applications.
To take such interfering fields into account, differential techniques have been proposed which, for example, comprise several Hall elements and link the corresponding sensor signals of the individual elements to one another in such a manner that they correspond to first and/or second derivatives of the magnetic field. A typical example of such a differential detector is a ratiometric sine/cosine encoder of the type used for multipolar magnetic strips. In such detectors, a ratio of the sine and cosine functions is usually formed and an angle or a position is derived. A magnetic field sensor will be referred to below as ratiometric if an output signal can be derived from a ratio of input parameters with the same interference superimposition. In the present case, the measured angle is formed by a ratio of the sine and cosine functions of the sensor signals. The two functions are dependent on an air gap, for example, which is the same for both parameters, however. Due to the formation of the ratio, the output signal becomes independent of such influences, or ratiometric.
The aforementioned detectors have the disadvantage, however, that they only supply a signal as long as a magnetic field source is located above the sensor. If it is removed, then the output signal of such detectors is undefined.